1. Field of the Invention
The present invention relates to an electrically operated effective for shifting a valve between open and closed positions, valve actuator comprising a movable member in the form of a valve stem connected to a valve body, the movable member being driven by an electric motor in linear motion between valve closing and valve opening positions, wherein electric power to the motor is supplied via a current control including a supplied current detection device.
2. Description of the Related Art
In the subsea industry, valves are traditionally activated by hydraulic devices. In a hydraulic actuator, end stop detection and control meet no problem as it is the nature of a hydraulic system. When the end of travel is reached, the valve stem stops and the hydraulic pressure is returned to normal working pressure.
In electrically operated valve actuators this is quite different. The torque of the motor is proportional to the current supplied. If the rotor of a motor is stopped or impeded, such as by the valve stem reaching a sudden stop at the end of travel, the current in the stator windings will increase to overcome the additional torque required to move the rotor. If the current supply to the stator is not controlled, the stator windings will burn and the motor is destroyed. A current limiter, which is typically electronically controlled, is required to avoid burning of the motor windings. The primary control may be achieved by a position sensor included in the control circuit, and which is effective for detecting the position of the valve stem. This approach is demonstrated in EP 0 287 299, e.g., wherein a microprocessor periodically tests the status of the valve stem of a motor operated valve, and turns off the valve's motor if certain predefined criteria for the valve stem position and the valve stem load are satisfied. However, if the position sensor is offset or otherwise fails, the current control is set aside and the motor looses protection from overheating.